Polyolefin fibers such as a polypropylene fiber are strong and lightweight, and are used widely in an industrial application, and the like. In order to reinforce the impact strength and the bending strength of a hydraulic cured body such as concrete and mortar, short fibers made of polypropylene, vinylon, and the like are used as reinforcing fibers. As the reinforcing fibers, fibers having excellent fiber strength and fiber elongation have been used. For obtaining such fibers having excellent strength and elongation, the selection of polymers, etc., has been performed. In particular, as reinforcing fibers for autoclave curing concrete, vinylon cannot be used because it decomposes, whereas polyolefin such as polypropylene has been put into practical use because of its stability. In some cases, a fiber treating agent (also called as a fiber finishing oil) is adhered to the cement reinforcing fibers for the purpose of uniformly dispersing reinforcing fibers in a hydraulic cured body. For example, Patent Document 1 describes cement reinforcing fibers to which a fiber treating agent containing an alkyl phosphate represented by the following chemical formula (1) as a main component has been adhered.

Further, as another method for uniformly dispersing reinforcing fibers in a hydraulic cured body, it has been studied to improve an affinity of fibers for a hydraulic cured body by hydrophilizing fibers through a hydrophilization treatment such as a fluorine treatment, a plasma discharge treatment, and a corona discharge treatment. For example, Patent Document 2 describes cement reinforcing fibers obtained by subjecting fibers to any one of a surface treatment selected from a corona discharge treatment, an atmospheric plasma treatment, and an ozone aqueous solution treatment, and then adhering a fiber treating agent to the surface-treated fibers.